Preparation And Performance Of Novel Perovskite Cathode Materials For Intermediate-temperature Solid Oxide Fuel Cells

The promise of direct and efficient conversion of chemical to electrical energy makes fuel cell development an area of great technological interest. The advantages over traditional power generation systems are numerous. Most prominent is the increased efficiency associated with directly converting chemical energy to electrical energy. Fuel cells are not subject to the Carnot-cycle limitations, and unlike high-efficiency turbines, exhibit their highest efficiency at low loads. Fuel cells do not produce significant quantities of NOx, SOx, or particulate pollutants.so Fuel cells be friendly to environment .Compared to normal batteries, fuel cells can have much higher energy densities and can be recharged more quickly and easily. Finally, fuel cells can be applied in applications that require both low and high power outputs, and they can be modular and portable.Solid oxide fuel cell (SOFC) is the fourth generation of fuel cells.compared to other fuel cells ,SOFC have many advantages: first, Conponents of SOFC are all solid state batteries, This avoid electrolyte corrosion and electrolyte loss and other problems that are brought when uses liquid electrolyte;Second,the working of SOFC at high temperatures,so the electrode response has been very rapid, without using precious metal as electrodes, so the battery cost greatly reduced; while the high quality exhaust heat of SOFC can be fully utilized, which can make utilization of energy from the simple 50% to 80%; More there is a wide range of SOFC fuel application, form the H2, CO and other fuels, to natural gas, liquefied gas and other hydrocarbons.These advantages make SOFC as a future star of green energy that have received a great of the world's attention.With the practical application of SOFC in further research, reducing the cost of the production and use is key,Effective way to reduce these costs is to make operating temperature of SOFC down to low temperature, but the lowering operating temperature will cause the cathode polarization resistance and electrolyte ohmic resistance increased significantly, resulting in significantly reducing performance of fuel cell. as the electrolyte thin film technology and high ionic conductivity of the electrolyte development,the losses of Ohmic resistance of electrolyte largely eased. so as to overcome the problems that caused by lower operating temperature ,wo ought to enhance the catalytic activity and lower cathode overpotential cathode. Cathode materials have a direct impact on the output performance of the fuel cell, so the development of high-performancecathode material of intermediate-temperature solid oxide fuel cell (IT-SOFC) is very important .the cathode materialof SOFC has not only have high catalytic activity for oxygen reduction, but also have good chemical thermal compatibility with the adjacent materials,and long-term stability. In this paper, electronic - ionic mixed conducting cathode materials and composite cathode materials were studied,researching the structure,thermal resistance and electrochemical properties,etc.of material , validating them as IT-SOFC cathode material is feasible.Numerous studies show LnBaCo2O5+δthat is the electronic and ionic mixed conductors have fast oxygen ion diffusion and surface exchange kineticsat at low temperature, so it is research's focus as IT-SOFC cathode materials,However,the short of LnBaCo2O5+δis thermal expansion coefficient much larger than the electrolyte, it will affect heat cycle of SOFC, so we use a larger ionic radius of Cu substitution for Co in NdBaCo2O5+δ.the NdBaCoCuO5+δcathode materials was synthesis via EDTA-citric acid method ,XRD analysis results show that the NdBaCoCuO5+δand NdBaCo2O5+δwith the same double perovskite with an orthorhombic structure.the conductivity of NBCC smaller than the conductivity of NBCO,the reason is the replacement of the smaller Co by the larger ionic radius Cu ,which leading to decline in the tolerance factor,in the crystal symmetry ,then a small polaron binding energy increase, the conductivity activation energy of small polarons increases, causing the conductivity decline. Partial substitution of Cu for Co will partially inhibit the Co3+ spin state changes from low to high spin state,so reducing the thermal expansion coefficient of the cathode material, an increase thermal matching of cathode materials and electrolyte .the polarization resistance of NBCC is only 0.027?cm2 at 800oC ,the NBCC showed a good microstructure. Using NBCC as the cathode, Ni0.9Cu0.1-SDC as anode, LSGM as electrolyte of the electrolyte supported single cell,the maximum output power density of the single cell wee 431.6mW/cm2 and 594.9mW/cm2 at 750oC, 800oC .Recently,BaCo0.7Fe0.2Nb0.1O3-δ(BCFN) perovskite oxide showed considerable oxygen permeability and good structural stability, is a good IT-SOFC cathode materials. However BCFN has high thermal expansion coefficient, so we added SDC electrolyte to BCFN cathode to improve the thermal matching. In this paper, BCFN cathode material was synthesized by solid-phase mothod, and then mixed BCFN with SDC by different mass ratio . The results show thatthere is no adverse reaction between BCFN and SDC,this indicated that there are very well chemical compatibility between them .Adding SDC to BCFN reduces the thermal expansion coefficient of the cathode material,improves the thermal compatibility between cathode and electrolyte , and improves the microstructure of the material, extends the length of three-phase interface and improves the electrochemical properties of the material. when the composite containing 30wt % SDC showed the best performance,the interface polarization resistance is 0.0104?cm2, which is much smaller than the interface polarization resistance of pure BCFN cathode (0.036?cm2) . Using Ni0.9Cu0.1-SDC as anode, BCFN-SDC as the cathode, LSGM as electrolyte of the electrolyte supported single cell,maximum power density were 557mWcm-2, 613mWcm-2, 644mWcm-2, 697mWcm-2, 680mWcm- 2 at 800oC. which indicates that BCFN-SDC is a promising IT-SOFC cathode materials. Due to GdBaCo2O5+δ(GBCO) rapid oxygen ion diffusion and good surface exchange kinetics at intermediate-temperature ,GBCO was widely studied as IT-SOFC cathode materials , and the results are good, but GBCO has high thermal expansion coefficient, in this paper,the Cu substitution for Co in GdBaCo2O5+δoxide is attempted with the purpose to modify the properties.its structure,thermal expansion coefficient and electrochemical properties were studied. after calcination at 1000oC for 5 hours ,GBCC exhibits a pure orthogonal structure of the double perovskite .After the Cu doping in B-sites,the thermal expansion coefficient decreased obviously,due to Cu substitution partially suppresses the transition of spin state Co3+ ,at 30oC ~ 850oC the thermal expansion coefficient of GBCC is 14.63×10-6K-1,which is much smaller than thethermal expansion coefficient of GBCO(20.1×10-6K-1) ,the TEC of GBCC is very close to the electrolyte.The interface polarization resistance of GBCC / LSGM is 0.042?cm2 at 800oC, so GBCC showed good catalytic cathode. With LSGM electrolyte of the electrolyte supported single cell GBCC | LSGM / SDC | Ni0.9Cu0.1-SDC ,the maximum power density of the cell reach 451.5mW/cm2,572.7mW/cm2 at750oC,800oC,which is comparably higher than the GBCO.In This paper ,the basic starting point is to reduce the thermal expansion coefficient and improve the performance of cathode materials, using doping and composite materials to reduce the thermal expansion coefficient of the cathode and improve the cathode performance, get a good result.